how to combine 2 or more textures in Cycles? 2.7xxx
You can combine textures using the MixRGB node(Add>Color>MixRGB in the Shader Editor):



Note that you can choose different blending types: 



As you can see you can mix any textures - proceedural and image. You can mix more textures by simply adding more MixRGB nodes.

In order to mix two textures with a smooth gradient you can use a gradient texture as a factor for the mix node:



I am using a MixRB node in Multiply mode to multiply the texture coordinate vector(to scale it) by 2 to make the gradient smaller. MixRGB node is for work with color, however it does work with any numbers as well and multiply mode does what it says - multiplies the values and it even supports negative values despite the fact that colors cannot have negative values, so there is nothing wrong to use it for some math with vectors as well.